One of the most remarkable features about the corpus luteum is that it is a transient gland: in the absence of conception, the corpus luteum disappears from the surface of the ovary at the end of each menstrual or estrous cycle. The mechanisms that are responsible for the spontaneous involution of the corpus luteum are largely unknown. In a number of species macrophages have been observed to accumulate in the regressing corpus luteum, and other immune cells are present suggesting that the immune system has an important role in the destruction of the corpus luteum. The biological signals that are responsible for the massive recruitment of monocytes/macrophages into the corpus luteum at the end of the cycle, and the mechanisms which prevent their recruitment during pregnancy are unknown. Based upon recent studies from this laboratory, in this proposal we will investigate the hypothesis that the accumulation of monocytes/macrophages in the rat corpus luteum is mediated by the chemokine, monocyte chemoattractant/activating protein l (MCP-l), and that the expression of this protein is essential for the physiological destruction of the corpus luteum. Using immunohistochemistry, Northern blot analysis, in situ hybridization, a MCP-l chemotaxis assay, and an assay for quantitation of MCP- l in extracts of corpora lutea, the temporal relationship between the expression of MCP-I and the appearance of monocytes/macrophages in the corpus luteum of pregnancy, pseudopregnancy, and of the estrous cycle will be determined. The physiological role of MCP-l will be investigated by intervention (injection) with anti-rat MCP-l antibody, to determine whether the recruitment of monocytes/macrophages can be blocked, and whether the administration of antibody prevents normal regressive changes such as the loss of tissue protein, steroidogenic capacity, aromatase activity, and 3B-hydroxysteroid dehydrogenase as the corpus luteum regresses in response to a specific physiological signal, prolactin. A new explanation is proposed for the paradoxical luteotrophic/luteolytic effect of prolactin which is based upon the hypothesis that luteal regression in the rat is mediated through an action of prolactin to stimulate the expression of MCP- I- leading to recruitment/activation of monocytes/macrophages, which then destroy the corpus luteum. From preliminary studies we now know that prolactin stimulates the expression of MCP-l and a massive invasion of macrophages in the rat corpus luteum. Using cycling rats in which regression of the corpus luteum is prevented by ergocryptine, immature hypophysectomized rats in which a first ovulation is induced and adult hypophysectomized rats in which the corpora lutea persist, the effects of prolactin administration will be investigated with respect to the induction of expression of MCP-I, the recruitment of monocytes/macrophages, and the induction of regressive changes in the corpora lutea. These experiments will test a new hypothesis that expression of MCP- l in the rat corpus luteum and the subsequent recruitment of monocytes/macrophages mediate the physiological destruction of the corpus luteum, and that in the rat, the expression of MCP-1 is regulated by prolactin.